THE PRIME VARIABLES OF MEIOSIS

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Abstract
Summary: 1. The uniform series of chromosome movements we know as meiosis (pairing, torsion, reproduction, crossing‐over, co‐orientation, segregation) can now be placed in a causal sequence. This serves two purposes: to be tested as a working hypothesis and to be applied to the understanding of the characteristic variations of which meiosis is susceptible.2. Comparison of mutants, hybrids and other genetically controlled variants shows that these variations arise from three main sources: (i) the point at which the pairing chromosomes make contact; (ii) the time available for pairing; (iii) the amount of torsion capable of being developed in the parts of the chromosomes which are paired.3. All species are, as such, characteristically co‐ordinated with regard to these variables. They may be classified as procentric (Mecostethus) or proterminal (Chry‐sochruon) in the initiation of pairing. This difference affects the relative frequency of crossing‐over in chromosomes of two main types, with the centromere near an end and away from it (Fritillariu, Lilium).4. Exceptional cells, and individuals arising by segregation from hybrids, show that co‐ordination within the cell and the individual is not physiologically inherent in meiosis. Unco‐ordinated behaviour can take place in regard to all three prime variables (Allium, Lilium and Trillium).5. The time limit to pairing may be imposed artificially by heat treatment or X‐raying. We then have artificial localization of crossing‐over (Uvularia, Vicia).6. The amount of torsion depends on the speed of pairing and this in turn on the size of the nucleus. Doubling the chromosome number therefore reduces the crossing‐over frequency (Primula, Allium, Solanum, etc.).7. The preservation of torsion depends on the early contact of a chromosome at two points. The more numerous contact points of triploids therefore increase their frequency of crossing‐over per unit length paired, at the same time changing its distribution (Fritillaria, Tulipa, Drosophila).8. All species hybrids are structural hybrids. Their pairing is therefore slower and is cut short by the time limit. Hence their crossing‐over is reduced and relatively localized, unless the extreme of localization has already been reached in the parents (Triticum, Lilium).9. The study of the three prime variables is therefore necessary for the understanding of the causal sequence of meiosis and of the conditions of stability and coherence in species.

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